Household Secondary Attack Rates of SARS-CoV-2 by Variant and Vaccination Status

This systematic reviewe and meta-analysis evaluates household secondary attack rates of SARS-CoV-2 by variant and vaccination status.

Reports excluded (n = 67): -No data on uninfected contacts (n = 22) -No data on household contacts (n = 19) -Reported prevalence or overall household attack rate, which includes index cases (n = 10) -Tested household contacts using antibody tests (n = 9) -No original data (n = 4) -Overlapping study population with another article included in meta-analysis (n = 2) -Restricted to households with at least one confirmed case among household contacts (n = 1) New studies included (n = 58)

Included
Total studies included in review (n = 135) Records identified from reference lists of eligible articles (n = 2) Studies included in previous version of review (n = 87 studies)

Previous studies
Reviewed methodology to exclude studies that did not include laboratory-confirmed infections, included asymptomatic index cases only, and were preprints that were subsequently published (n = 87) Records excluded (n = 10) -No lab-confirmed infections (n = 5) -Preprints in first analysis that were subsequently published (n = 4) -Asymptomatic index cases (n = 1)

eTable 1. Electronic Databases and Search Strategy for Household Secondary Attack Rate of SARS-CoV-2
Database  Figure 2 when restricting to studies with low risk of bias. Received a booster dose was defined as having received an additional dose after completion of the primary COVID-19 vaccination series before the index date. Fully vaccinated was defined as completion of the primary vaccination series ≥2 weeks before the index date and stratified into completion <5 months or ≥5 months before the index date. Some persons who were fully vaccinated had unknown dates for completion of their primary vaccination series. Partially vaccinated was defined as having only 1 dose of a 2-dose series or completing the primary vaccination series <2 weeks before the index date.

eTable 5. Pairwise Analyses of Index Case Vaccination Status Using Only Studies in Which SARs Were Reported From Both Relevant Subgroups
De Gier et al. 11 Partly vaccinated was defined as having received the first dose of a two-dose schedule at least 14 days before onset of symptoms. Fully vaccinated was defined as having completed a two-dose schedule at least 7 days or the one-dose Janssen schedule at least 14 days before symptom onset.  13 Vaccination status was dichotomized to either non vaccinated or fully vaccinated as per each vaccine's protocol.

Harris et al. 1
Vaccinated index cases defined as having been vaccinated 21 days or more prior to testing positive for COVID-19 based on evidence of the time needed for the vaccine to provide a sufficient level of immunity. Non-vaccinated index cases were defined as not having received a vaccine prior to testing positive. Households where the index case received the vaccine less than 21 days before testing positive were excluded from this analysis. Most of the vaccinated index patients (93%) had received only the first dose of vaccine.

Gazit et al. 17
Participants were classified into one of three vaccination-status groups at the time of the index case (the confirmed exposure): Unvaccinated; Recently Vaccinated Once, i.e. those vaccinated with the first vaccine dose within 0-7 days before the index infection, and Fully Vaccinated, i.e. those who were 7 or more days post the second dose by the time of the confirmed exposure.

Jalali et al. 22
To define the vaccine status of the household contacts, they used the test date of the primary case and compared it with the contacts vaccination dates: 1.Unvaccinated: A contact was considered unvaccinated if the primary case's test date is before the contact's first dose. 2.Partially vaccinated: A contact was considered partially vaccinated if he/she had received 1 dose of vaccine (mRNA Vaccines or AstraZeneca vaccine) prior to the test date of his/her primary case. Contacts who had received dose 2 within the last week before the primary case's test date were also considered partly vaccinated. 3.Fully vaccinated: A contact was considered fully vaccinated if he/she had received dose 2 (mRNA) at least 1 week prior to the test date of his/her primary case. 4.Booster vaccinated: A contact was considered booster vaccinated if he/she had received dose 3 at least 1 week prior to the test date of his/her primary case. The time interval between the second and the third doses should be >= 120 days. The vaccine status of the primary cases was defined based on their test date and their vaccination dates. Individuals with J&J vaccine were excluded from the study. We excluded households where two individuals tested positive on the same day to ensure a unique index case in each household.

Layan et al. 25
Cases were considered vaccinated if their infection occurred >7 days after the 2nd dose. Similarly, household contacts were considered vaccinated if their exposure to the index case occurred >7 days after the 2nd dose Martínez-Baz et al. 34 A person was considered fully vaccinated ≥ 14 days after receiving one dose of Janssen or the second dose of other vaccines, and partially vaccinated ≥ 14 days after receiving only the first dose of Spikevax, Comirnaty or Vaxzevria.
Meyer et al. 36 Not defined, but the two secondary cases found among household contacts of vaccinated index cases were diagnosed 25 days after the second vaccination.

Ng et al. 41
Both index cases and close contacts were considered partially vaccinated if they had received one vaccine dose before the day the quarantine order was issued, or were within 14 days of the second dose on the day the quarantine order was issued. If more than 14 days had elapsed after their second dose, they were taken to be fully vaccinated.

Sachdev et al. 47
Partially vaccinated patients were defined as patients who received at least 1 dose of vaccine but were not fully vaccinated. Fully vaccinated patients were defined as patients who had received a second mRNA vaccine dose or a single-dose viral vector vaccine ≥14 days from symptom onset or collection of a positive specimen Singanayagam et al. 48 Participant defined as unvaccinated if they had not received a single dose of a COVID-19 vaccine at least 7 days before enrolment, partially vaccinated if they had received one vaccine dose at least 7 days before study enrolment, and fully vaccinated if they had received two doses of a COVID-19 vaccine at least 7 days before study enrolment.